The process variable is preferably volume- or mass-flow, e.g. flow rate, of a medium through a pipeline or canal. Corresponding ultrasonic flow measuring devices are offered and sold by Endress+Hauser. Furthermore, the process variable can also be the fill level of a fill substance in a container, as measured by means of an ultrasonic travel time method. In general, the method of the invention is applicable for all measuring devices in which ultrasonic measuring signals are transmitted and received.
Without intending to be limited thereto, in the following, reference will be to an ultrasonic flow measuring device. An inline ultrasonic flow measuring device is usually integrated into a pipeline in which a medium to be measured is flowing. Ultrasonic flow measuring devices working according to the travel-time difference method include at least one pair of ultrasonic sensors, which transmit and/or receive ultrasonic measuring signals along defined sound paths. A control/evaluation unit ascertains the volume- and/or mass-flow of the measured medium in the pipeline on the basis of the difference of the travel times of the measuring signals in the flow direction of the measured medium and opposite to the flow direction of the measured medium. The measured medium can be a gaseous or a liquid medium.
Besides the above-described, inline ultrasonic flow measuring devices, also clamp-on flow measuring devices are used, which are mounted externally on the pipeline and measure volume- or mass-flow by transmitting sound through the pipe wall.
Ultrasonic flow measuring devices of the above described kind, which ascertain volume- or mass-flow, are applied often in process and automation technology. Clamp-on flow measuring devices have the advantage that volume- or mass-flow can be ascertained in a containment, e.g. in a pipeline, without contact with the medium. Clamp-on flow measuring devices are described, for example, in EP 0 686 255 B1, U.S. Pat. No. 4,484,478, DE 43 35 369 C1, DE 298 03 911 U1, DE 4336370 C1 or U.S. Pat. No. 4,598,593.
In the case of both types of ultrasonic flow measuring devices, the ultrasonic measuring signals are radiated at a predetermined angle, into, respectively out of, the pipeline. In these measuring devices, the respective positions of the ultrasonic transducers on the measuring tube (inline), or on the pipeline (clamp-on), depend on the inner diameter of the measuring tube and the velocity of sound in the measured medium. In the case of a clamp-on flow measuring device, additionally the application parameters wall thickness of the pipeline and velocity of sound in the material of the pipeline must be taken into consideration.
Usually, in the case of both types of ultrasonic flow measuring devices, the ultrasonic sensors are so arranged that the traversing sound paths are directed through the central region of the pipeline or measuring tube. The ascertained measured-value for the flow reflects, thus, the average flow of the measured medium. In many applications, especially in the case of flow measurements in pipelines of large nominal diameter, this averaging is, however, too inexact. Therefore, it has also become known to provide a plurality of sensor pairs distributed over the periphery of the measuring tube or pipeline, so that flow information is available for various segmented, angular regions of the measuring tube or pipeline.
The essential component of an ultrasonic sensor is a piezoelectric element. The essential component of a piezoelectric element is a piezoceramic layer, in the form of a film, membrane or diaphragm. The piezoceramic is metallized at least in a portion of a region. By application of an electrical, exciting signal, the piezoelectric layer is caused to oscillate. An ultrasonic sensor would function ideally, if the piezoelectric element would exactly follow the electrical, exciting signal—this is, however, not the case in the real world. Rather, the exciting signal usually also excites harmonics and resonances, which are superimposed on the wanted signal and change its signal-, or wave-, form. In this way, the accuracy of measurement of a measuring method based on the piezoelectric effect can be significantly limited.